1. Field of the Invention
This invention relates to optical couplers and, more particularly, to optical couplers for coupling multimode light contained in optical fibers to planar optical waveguides.
2. Description of the Prior Art
Optical communications systems provide large bandwidths and the number of message channels that may be realized and the rapidity with which information may be transmitted far exceeds that obtainable with more conventional electromagnetic transmission systems. To realize such a communications system low loss fiber optic waveguides and optical processing devices have been developed. Fiber optic waveguides transmit the light contained therein in a multiplicity of optical modes while the processing devices generally can handle but a single mode. Consequently, the light energy contained in but a small percentage of the propagating modes of a fiber optic waveguide is coupled to the processing device and an extremely high energy loss is realized.
Optical switches and modulators have been proposed that can propagate multimode energy. One such device takes the form of a voltage induced planar optical waveguide as disclosed by Channin in U.S. Pat. No. 3,795,433. The relatively small change in refractive index produced electro-optically in this device is insufficient to handle the multimode light energy exiting any single multimode fiber without an effective means for multimode coupling. Though efficient techniques have been proposed for optically coupling light wave energy between planar optical waveguides and optical fiber waveguides, such as the techniques disclosed by Hammer in U.S. Pat. No. 3,912,363 and by Smolinsky et al in U.S. Pat. No. 3,864,019, these have provided coupling primarily for a single mode or for a relatively small percentage of the modes of the multimode fiber. No single prior art device has been disclosed that both efficiently couples to an optical waveguide all of the light emitted by a single multimode fiber and also permits efficient switching of an appreciable fraction of this multimode energy with voltages well below the electro-optic breakdown voltage.